Scope and Mechanism of a True Organocatalytic Beckmann

Mar 22, 2018 - Using only 5 mol % of boronic acid catalyst and perfluoropinacol as an additive in a polar solvent mixture, the operationally simple pr...
76 downloads 4 Views 2MB Size
Download PDF
Article Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

pubs.acs.org/JACS

Scope and Mechanism of a True Organocatalytic Beckmann Rearrangement with a Boronic Acid/Perfluoropinacol System under Ambient Conditions Xiaobin Mo,‡ Timothy D. R. Morgan,‡ Hwee Ting Ang, and Dennis G. Hall* Department of Chemistry, University of Alberta, Edmonton, Alberta T6G 2G2, Canada S Supporting Information *

ABSTRACT: Catalytic activation of hydroxyl functionalities is of great interest for the production of pharmaceuticals and commodity chemicals. Here, 2-alkoxycarbonyl- and 2-phenoxycarbonyl-phenylboronic acid were identified as efficient catalysts for the direct and chemoselective activation of oxime N−OH bonds in the Beckmann rearrangement. This classical organic reaction provides a unique approach to prepare functionalized amide products that may be difficult to access using traditional amide coupling between carboxylic acids and amines. Using only 5 mol % of boronic acid catalyst and perfluoropinacol as an additive in a polar solvent mixture, the operationally simple protocol features mild conditions, a broad substrate scope, and a high functional group tolerance. A wide variety of diaryl, aryl-alkyl, heteroaryl-alkyl, and dialkyl oximes react under ambient conditions to afford high yields of amide products. Free alcohols, amides, carboxyesters, and many other functionalities are compatible with the reaction conditions. Investigations of the catalytic cycle revealed a novel boron-induced oxime transesterification providing an acyl oxime intermediate involved in a fully catalytic nonself-propagating Beckmann rearrangement mechanism. The acyl oxime intermediate was prepared independently and was subjected to the reaction conditions. It was found to be self-sufficient; it reacts rapidly, unimolecularly without the need for free oxime. A series of control experiments and 18O labeling studies support a true catalytic pathway involving an ionic transition structure with an active and essential role for the boronyl moiety in both steps of transesterification and rearrangement. According to 11B NMR spectroscopic studies, the additive perfluoropinacol provides a transient, electrophilic boronic ester that is thought to serve as an internal Lewis acid to activate the ortho-carboxyester and accelerate the initial, rate-limiting step of transesterification between the precatalyst and the oxime substrate.

■

INTRODUCTION New catalytic modes of functional group activation can unlock unique reactivity in organic molecules and lead to novel bondforming processes. When applied to safe and readily available substrates such as alcohols and carboxylic acids, efficient and atom-economical methods can be developed that bypass the use of toxic halide derivatives or stoichiometric reagents.1 In this regard, the concept of boronic acid catalysis (BAC) exploits the tempered Lewis acidity of boronic acids along with their ability to form reversible covalent bonds with hydroxyl functionalities, thereby providing an opportunity for temporary activation of C−O bonds. Several applications of BAC have been demonstrated by our group and others.2 For instance, we reported direct boronic acid catalyzed transposition3 and substitutions4 of allylic alcohols, and Friedel−Crafts alkylation of neutral arenes with readily available allylic and benzylic alcohols.5 With a view to expand BAC toward other dehydrative reactions of hydroxyl-containing functional groups, we considered the direct Beckmann rearrangement of ketoximes into secondary amides.6 In its classical form, the Beckmann rearrangement is promoted by strong protic acids.7 © XXXX American Chemical Society

This important transformation is at the heart of the industrial synthesis of lactam monomers for the manufacture of nylon, and it also finds utility in the discovery and production of pharmaceuticals.8 Mild catalytic manifolds are rare, and oftentimes, preactivation of the hydroxyl (i.e., tosylation) is required. Compared to the use of inorganic Lewis acids, organocatalysis offers notable advantages such as broader functional group compatibility and operational simplicity (i.e., air and moisture tolerance). Reported procedures for the Beckmann rearrangement employing organic compounds as promoters, such as cyanuric chloride,9 BOPCl,10 triphosphazene,11 tosyl chloride,12 propylphosphonic anhydride (T3P),13 TFA,14 or cyclopropenium salts15 require an acid cocatalyst or elevated temperatures (60− 100 °C) to achieve effective activation of oximes (Figure 1). Many of these reagents have been shown to act merely as initiators of the Beckmann rearrangement through a selfpropagating mechanism involving a nitrilium intermediate and Received: February 9, 2018 Published: March 22, 2018 A

DOI: 10.1021/jacs.8b01618 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Article

Journal of the American Chemical Society

Table 1. Optimization of Boronic Acid Catalyzed Beckmann Rearrangement

entrya

cat.

mol (%)

x:y

[M]

T (°C)

t (h)

3ab (%)

1 2 3 4 5 6 7c 8d 9d 10d 11f 12

1a 1a 1a 1a 1a 1a 1a 1a 1a 1b 1a 1c (R = H)

10 10 10 10 10 10 10 5 5 5 5 20

1:4 1:2 1:1 4:1 1:4 1:4 1:4 1:4 1:4 1:4 1:4 1:4

0.5 0.5 0.5 0.5 1.0 1.0 1.0 1.0 1.0 1.0 1.0 0.5

50 50 50 50 50 rt rt rt rt rt rt 50

72 72 72 72 72 24 24 24 6 6 24 72

100 93 79 60 87 72 94 93e 37e 94 86 0

Figure 1. Classical and organo-mediated/catalyzed Beckmann Rearrangement procedures. a

Reaction conditions: 0.5 mmol of oxime were dissolved in the indicated solvent mixture. bYields were determined by 1H NMR analysis of reaction mixture using 1,4-dinitrobenzene as an internal standard. cReaction was run with 10 mol % of perfluoropinacol. d Reaction was run with 5 mol % perfluoropinacol. eIsolated yield. f50 mol % of water was added to the solvent.

are, therefore, not true catalysts.12,15 These limitations, coupled with the significance of the Beckmann rearrangement as a unique method for the synthesis of amides, warrant the development of new organocatalytic procedures that can operate at or near ambient temperature for a wider variety of substrates. Herein, using the BAC concept, we report a direct Beckmann rearrangement procedure that displays high functional group tolerance under ambient conditions for diaryl, arylalkyl, heteroaryl-alkyl, and dialkyl oximes, affording high yields of amide products.

appears to tolerate trace water (entry 11 vs entry 8), and the use of molecular sieves was found to have a negligible effect (see SI). Solvents need no exhaustive drying with this simple experimental procedure. Interestingly, 2-carboxyphenylboronic acid (1c) is ineffective (entry 12). Examination of Substrate Scope. A representative panel of 36 aromatic and alkyl substituted oximes were tested in the boronic acid catalyzed Beckmann rearrangement under the optimal conditions from Table 1 (entry 8 or 10) using 1a or 1b as the catalyst. As shown in Scheme 1, in most cases the reaction proceeded efficiently at room temperature for both aryl and alkyl substituted substrates. As demonstrated with 3a, this BAC procedure maintains its efficiency on gram scale. A range of functional groups were found to be fully tolerant of these reaction conditions. A phenol-containing substrate underwent the rearrangement in 93% yield (2k). As shown with products 3l−3r, halide, alcohol, amide, carboxyester, and nitrile groups are compatible. Moreover, a variety of common protecting groups were also tolerated, including acid sensitive Boc (2j) and Ts (2aa) groups. Pharmaceutically relevant heteroaromatic substrates also reacted efficiently, including the use of free pyrrole (2y) and indole (2z). These unprotected substrates offer a significant synthetic advantage, in terms of step and atom economy, over amidation methods that require protected heterocycles. Hindered heteroaromatic amides such as 3y are unprecedented and would be difficult to prepare using standard amide coupling methods. The temperature sensitive enamide 3ab was obtained in high yield under ambient conditions with this procedure. The steroid pregnenolone oxime 2aj underwent the Beckmann rearrangement in 76% yield even without protection of the alcohol. Cyclohexanone oxime 2ag, a notoriously challenging substrate,9−12,15 gave a 65% yield under more forcing conditions. Additionally, the superiority of catalyst 1b over 1a is clearly evidenced by the examples of oximes 2l, 2u, and 2af. As expected, aromatic

■

RESULTS AND DISCUSSION Optimization of Reaction Conditions. The initial optimization focused on the identification of effective boronic acids capable of mediating the prototypic Beckmann rearrangement of oxime 2a into amide 3a (Table 1). Over 20 functionalized aryl boronic acids16 were screened in different solvents (see Supporting Information (SI)). Only 2-methoxycarbonylphenyl boronic acid (1a) provided a significant amount of the amide product. It was found to transform oxime 2a into amide 3a in good yield in a solvent mixture of 4:1 hexafluoroisopropanol (HFIP) and nitromethane at 50 °C after 72 h (entry 1). The use of HFIP is vital to the reaction, and a higher conversion is achieved with a higher proportion of HFIP (entries 2−4). Further optimization showed that the concentration could be increased up to 1.0 M, thus providing substantial solvent economy without undermining the reaction yield (entry 5). However, the reaction exhibited lower efficiency when decreasing the reaction time and temperature (entry 6). We hypothesized that, in addition to providing a polar reaction medium, HFIP could form an electron-poor boronic ester and modulate the Lewis acidity of the boron atom. To this end, a catalytic amount of perfluoropinacol was added to form a more stable cyclic boronic ester. Satisfactorily, product 3a formed in higher conversion at room temperature after 24 h (entry 7). With perfluoropinacol, the catalyst loading could be decreased to 5 mol %, affording 3a in high yield (entry 8). Further optimization of the boronic acid (see SI) led to the use of phenoxy ester 1b, which considerably shortened the reaction time providing 3a in high yield after only 6 h at room temperature (entries 9−10). It is noteworthy that the reaction B

DOI: 10.1021/jacs.8b01618 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Article

Journal of the American Chemical Society

elevated temperature, no product was observed when the highly deactivated oxime 2s was used under various conditions. Oxime 2w was also unreactive, presumably due to catalyst inhibition from chelation of the boronic acid to the ortho-amine and the oxime moieties. Several of the oximes used in this study were formed as mixtures of E/Z isomers. Notably, in almost all cases the mixture of oximes resolved to a single major product with generally excellent selectivity based on the expected migratory aptitude of the R1 and R2 groups. The nonstereospecific nature of this Beckmann rearrangement method probably originates from the rapid interconversion of E and Z oximes under the reaction conditions.17 Due to the ability of HFIP to facilitate ionic reactions18 the BAC conditions were compared with some of the most active Beckmann rearrangement promoters reported. When T3P and TFA were applied to a subset of oxime substrates, the results were clearly inferior to the new BAC method (Scheme 2). In

Scheme 1. Substrate Scope of Boronic Acid Catalyzed Beckmann Rearrangementa

Scheme 2. Comparison of Organocatalysts for the Beckmann Rearrangement in CH3NO2/HFIPa

a

Reactions performed on 0.5 mmol scale. Yields are reported after isolation unless otherwise noted. bReaction was complete after 6 h.

fact, rearrangement of even the simplest of oximes, 2a, failed with the use of catalytic or even stoichiometric amounts of TFA.19 These results demonstrate that the exceptional activity of boronic acid catalysts 1a/1b is not simply due to the unusual solvent system employed or to the presence of adventitious acid. Chemical Orthogonality with Alcohols. Electron-poor arylboronic acids are excellent catalysts for the activation of certain alcohols in Friedel−Crafts reactions.5 We were interested in evaluating the potential of 1a to act as a chemoselective activator of oxime N−OH units. To this end, boronic acid 1a and oxime 2a were subjected to Friedel−Crafts benzylation conditions with p-xylene and alcohol 4 as competing substrates (Scheme 3).5b Remarkably, only the amide 3a was formed along with a good recovery of 2a and alcohol 4. None of the expected Friedel−Crafts product (5) was observed. This result highlights the potential of BAC in orthogonal catalysis of reactions with substrates containing multiple hydroxyl-containing functional groups.

a

Reactions performed on 0.5 mmol scale. Yields are reported after isolation unless otherwise noted. bGram scale, 30 h. cOxime was formed as a mixture of isomers. For ratios, see SI; however, unless noted, a single amide product was observed. dThe minor product resulted from migration of R2. eYield was determined by 1H NMR analysis of the reaction mixture with 1,4-dinitrobenzene as internal standard. f50 °C. g80 °C. hWith 10 mol % boronic acid and diol. iWith 30 mol % boronic acid and diol. j20 mol % boronic acid, CH3NO2/ HFIP (1:1), 50 °C, 24 h. kHFIP, 50 °C, 24 h.

oximes with electron-withdrawing substituents are less reactive due to the inferior migratory aptitude of these groups. Although carboxyester-substituted aromatic ketone 2r was successful at C

DOI: 10.1021/jacs.8b01618 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Article

Journal of the American Chemical Society

conditions, 72% of amide 3a was observed (Scheme 3b). Altogether these results indicate that a Lewis acidic boronate can facilitate the downstream Beckmann rearrangement, but a free boronic acid is preferred for effecting the initial oxime transesterification step. When oxime ester A was exposed to the reaction solvent, spontaneous rearrangement occurred, forming 3a and a mixture of the carboxylic acid and HFIP ester of the catalyst. The self-sufficiency and high reactivity of A support a unimolecular Beckmann rearrangement that does not require a second molecule of oxime as needed in the self-propagating “nitrilium” mechanism.12,15 Studies of the Role of HFIP and Perfluoropinacol. Use of HFIP as solvent has previously been shown beneficial for cationic processes such as the Beckmann rearrangement.17 In addition to its role as a polar solvent, we suspected that HFIP could also engage in reversible boronic ester formation with catalyst 1a thus providing a highly electrophilic and Lewis acidic boron center (see SI). This idea led to the use of cocatalytic perfluoropinacol, which presumably forms a more stable five-membered boronic ester (Table 1, entries 7−10). The partial formation of a reactive perfluoropinacol boronate is supported by NMR studies (Figure 2). Mixtures of boronic acid 1a (δ = 30 ppm) and increasing amounts of perfluoropinacol in nitromethane-d3 were subjected to 11B NMR measurement (eq

Scheme 3. Chemoselectivity of Boronic Acid Catalyst 1a

Investigation of the ortho-Boronyl Group. Preliminary experiments were performed in order to shed light on the mechanism. The unique structure and reactivity of 2-methoxy/ phenoxycarbonyl-phenylboronic acids 1a and 1b led us to propose the formation of an o-boronyl oxime ester (A in Scheme 4c) as the key intermediate for this Beckmann Scheme 4. Mechanistic Control Experimentsa,b

a

Reaction conditions: 0.5 mmol of oxime was dissolved in the indicated solvent mixture without further precautions. bYields were determined by 1H NMR analysis of reaction mixture using 1,4dinitrobenzene as internal standard. cReaction was run with the conditions of Table 1, entry 10.

rearrangement procedure. Oxime esters were shown by Kuhara in the early 1900s to be suitable precursors in the Beckmann rearrangement.20 To support the intermediacy of an oxime ester, several control experiments were conducted. 2-Carboxyphenylboronic acid (1c) was found to be inactive under various conditions (see Table 1 and SI). However, when the reaction was performed in the presence of an esterification reagent to form the acyl oxime in situ, a moderate (52%) yield of amide product 3a was observed (Scheme 4a). Removing or replacing the boronyl group with other electron-withdrawing groups (e.g., NO2 and CF3) led to no product formation, which suggests a Lewis acidic role for the ortho-boronyl. Oxime ester A was prepared independently and shown to efficiently catalyze the rearrangement, which supports its intermediacy (Scheme 3b). Protecting catalyst 1a with pinacol before use resulted in only a trace amount of product (see SI). However, when the pinacol boronate of A was subjected to the standard reaction

Figure 2. Titration of boronic acid 1a with various amounts of perfluoropinacol via 11B NMR. D

DOI: 10.1021/jacs.8b01618 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Article

Journal of the American Chemical Society 1). A large shift in the 11B NMR from 30 to 15 ppm supports the presence of a tetrahedral boron atom arising from the coordination between the carbonyl and the Lewis acidic boron. The perfluoropinacol boronate was prepared independently from 1a (eq 2), and it was also subjected to NMR analysis. The same characteristic 11B NMR shift of 15 ppm confirms its formation in the titration studies. Downfield shifts in the 1H and 13C NMR signals for the carbonyl and methoxy groups also suggest an increase in the electrophilicity of the ester (Scheme 5a). These interactions are likely responsible for facilitating the transesterification and promoting the subsequent Beckmann rearrangement.

mechanism. The innate reactivity of A does not support a stepwise bimolecular self-propagating mechanism involving the reaction of a free nitrilium ion with oxime substrate.21 This sort of stepwise process has been previously shown to lead to dimerized side products with other organocatalysts.9,12b These byproducts were not observed when 1a was used as the catalyst with cyclohexanone oxime, which further supports the proposed mechanism when using 1a/1b as catalytic species.22 Although the role of the boronyl unit in the actual rearrangement is unclear, it is reasonable to assume that carboxyl coordination to the electrophilic boronate turns the benzoyl unit into a very reactive leaving group and thus facilitates the bond migration process (see TS in Scheme 5b). Other electron-withdrawing substituents such as nitro (cf. Scheme 4a) are incapable of this sort of internal Lewis acid promoted activation. The rearranged acyl imidate B eventually releases the amide product (D) via exchange with another molecule of oxime. Acyl imidate B is expected to be a significantly more reactive ester compared to 1a/1b and acyl oxime A. Thus, it reacts rapidly by transesterification with oxime substrate to recycle the effective catalyst, acyl oxime A. When no more oxime is available, esterification with HFIP, a much weaker nucleophile, leads to C, which was detected by MS and 1H NMR (see SI). A qualitative reaction progress kinetic analysis of the rearrangement step was performed by monitoring the decomposition of acyl oxime A into products C and D by LC-MS and UV detection (Figure 3). This study confirmed the rapid transformation of acyl oxime (A) into the amide and HFIP ester products at ambient temperature. Decay of A does

Scheme 5. Proposed Catalytic Cycle of Boronic Acid Catalyzed Beckmann Rearrangement

Mechanistic Proposal: True Catalysis vs Self-Propagation. Based on the above experiments, a catalytic cycle is proposed in Scheme 5. The catalyst undergoes boronic ester formation with perfluoropinacol resulting in a more electrophilic boron center, which mediates the oxime transesterification by activating the carboxyester and affording catalytic intermediate A (Scheme 5a). This boron-assisted oxime transesterification is likely to be the rate-limiting step, a notion supported by the superiority of precatalyst 1b with a better leaving group (OPh). Based on the ability of A to rearrange spontaneously and quantitatively in the absence of free oxime (cf. Scheme 4c), this boronic acid induced Beckmann rearrangement likely proceeds via a fully catalytic, unimolecular

Figure 3. Reaction progress kinetic analysis of the model Beckmann rearrangement of the acyl oxime (A) of acetophenone. Relative peak areas on the graph are not normalized. E

DOI: 10.1021/jacs.8b01618 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Article

Journal of the American Chemical Society Scheme 6. Rearrangement of 18O-Labeled Acyl Oxime A*

being incorporated into the amide product, not water, and it also rules out the self-propagating pathway commonly operative with other organocatalysts. Control analysis with authentic 2carboxyphenylboronic acid E (1c) confirmed that if it formed, carboxylic acid E or its boronate form would be easily detected under the LC-MS conditions utilized (see SI). To confidently rule out the self-propagated mechanism, an assumption deduced by the absence of 2-carboxyphenylboronic acid (E) from the reaction mixture, it was necessary to confirm that E is unable to convert to C under the reaction conditions. Indeed, subjecting 2-carboxyphenylboronic acid (E/1c) with and without oxime 2a in HFIP did not lead to the formation of the HFIP ester C (Scheme 7).

not show the linear relationship expected of a zero-order rearrangement. This observation is most likely explained by the involvement of HFIP as a reactant in the probable rate-limiting transesterification of the acyl imidate intermediate (B), a step required to release C and D. When used as solvent, HFIP thus leads to a pseudo-first-order situation that is reminiscent of the exponential profiles of Figure 3. To further support the proposed Beckmann rearrangement mechanism, an isotope labeling study was conducted using 18Olabeled acyl oxime (Scheme 6). If the rearrangement is unimolecular with ionic character, the label should distribute about equally into amide product D* and HFIP ester C*. If the rearrangement step is concerted, depending on the migrating oxygen of the carboxylate unit, the label should be observed exclusively in amide D* or, if 1,3-migration of the carboxylate is preferred, it will be observed in HFIP ester C*. With a selfpropagating pathway, the label should be incorporated solely into carboxylic acid E* (1c). Acyl oxime A* was synthesized as a 2,2-dimethylpropanediol boronate with an 18O label onto the sp3 oxygen. It was prepared with 51% 18O incorporation using a 18 O-labeled acetophenone oxime 2a* made according to a literature procedure.23 The results of this experiment, which was analyzed by HPLC-MS (selected ion monitoring mode), are shown in Scheme 6. Acyl oxime B* was treated in HFIP in the presence of a small amount of water to allow in situ boronate hydrolysis to the more reactive boronic acid. In our reaction optimization (cf., Table 1), water was shown not to significantly affect this Beckmann rearrangement procedure. Moreover, the presence of water in this investigation would allow us to confirm or rule out adventitious water as being the origin of the oxygen atom in the amide product. In the event, with A* normalized as 100% 18O, less than 6% loss of the 18O label was observed, and the 18O ended up partitioning into HFIP ester C* (35%) and amide product D* (59%). A negligible amount (